This invention relates to separation devices of the type used to fasten, retain or latch together components of apparatus or structure which are to be separated or released under controlled conditions. In particular, the invention relates to a non-explosive separation device for applications such as providing a safe and reliable replacement for explosion-actuated fasteners or explosive bolts.
Explosion-actuated separation devices such as explosive bolts have been used in a wide variety of applications in which components or parts of a device or structure are secured together for quick separation when the explosive device is activated. Explosive bolts of this type have been utilized for the deployment of payloads of space vehicles, for the safing and arming of ordnance and for the release of emergency hatches. They have also found application for the underwater release of elements, for example oceanographic equipment or for the recovery of instrument packages from torpedoes used in target practice.
While the explosive bolt technology has been developed to a high degree, their use for many applications presents a number of problems and limitations. When explosive bolts are actuated the resulting mechanical shock can cause unintended damage to associated structures, such as protective shrouds and fairings, and to equipment, for example to the electronics in payloads or to instrument packages of the components being separated. The explosion devices are also relatively sensitive such that they are hazardous to store and use. When actuated shrapnel from the explosive devices can cause injury to personnel and damage to surrounding equipment.
Another disadvantage from existing explosion-actuated separation devices is that there is no reliable non-destructive method for testing the integrity of the separation device in a stand-by mode. Furthermore, the deflagrating charges employed in such explosion-actuated devices are subject to deterioration over time with the result that they do not have a long shelf life. Explosive bolts are also relatively difficult to install in that special tools and trained personnel are normally required for safely installing and wiring the devices.
Among the prior art separation devices is the mechanism described in U.S. Pat. No. 3,454,286 which employs a fusible material that melts upon heating from ignition of a combustible material to release the elements that are being held together. While such a device is relatively safer in operation as compared to explosive bolts, it also presents a number of disadvantages and limitations such as limited shelf life of the combustible material and the limitations on testing of the actuating circuit and predicting the precise release point of the device.
A further prior art releasable latch mechanism is disclosed in U.S. Pat. No. 3,546,996 where a rise in temperature arms the device through a differential in thermal expansion of two elements. A subsequent drop in temperature causes thermal contraction of the elements back to ambient which ruptures one of the elements to release the latch. Such a mechanism also presents a number of disadvantages and limitations. Operation of the mechanism is dependent upon the relatively low coefficient of thermal expansion of the metal elements. This requires a large temperature excursion, and consequently a relatively long time period, to build up the requisite tension forces. Additionally, such a mechanism has poor operating sensitivity with the result that it is difficult to predict and control the point in the thermal cycle where the rupture will occur. Consequently such latch mechanisms have only a limited scope of application and generally are not adaptable as replacements for explosive bolts of the type now in use.
It is therefore a general object of the present invention to provide a separation device and method of operation which secures components of an apparatus together for non-explosive separation under controlled conditions.
Another object is to provide a separation device and method of the type described in which a shape-memory alloy material operates in a manner to separate the retaining element of the device into exactly two pieces.
Another object is to provide a non-explosive separation method of the type described using the 3 percent elongation of a shape memory alloy actuator to cause a 20 percent or more strain in the strain concentrated portion of the retaining element.
Another object is to provide a separation device and method of the type described in which, at the time of separation, the energy for operating the shape-memory alloy material is relatively small and is released over a relatively long time span as compared to the large energy release over a very short time span in an explosion separation device.
Another object is to provide a separation device and method of the type described which safely operates in a manner to minimize unintended damage to associated equipment and to also minimize risk of injury to personnel.
Another object is to provide a separation device and method of the type described which safely captures the two parts of the retaining element that are separated following actuation.
Another object is to provide a separation device of the type described which is durable and is not subject to deterioration over time such that it has a relatively long shelf life.
Another object is to provide a separation device of the type described in which the shape-memory alloy actuator can be operated an indefinite number of times so that the separation device is reusable upon replacement of the retaining element.
Another object is to provide a separation device of the type described which is compact, exerts a relatively high separation force for its size and which can be easily installed without the requirement of special tools.
Another object is to provide a separation device of the type described which is reliable in operation by being triggered through a simple heat source, such as an electrical or chemical heat source.
Another object is to provide a separation device and method of the type described in which the shape-memory alloy material is a part of the circuit with a current overload generating heat which causes separation of an element which opens the circuit.
Another object is to provide a separation device and method of the type described in which a relatively long and thin shape-memory alloy element is operated by a conventional electrical power source with the resulting force multiplied by means of lever action to cause a retaining element to separate into two parts.
Another object is to provide a separation device of the type described by which the separation time following activation of the heating phase can be predicted with accuracy.
Another object is to provide a separation device of the type described with lower, and therefore safer, activating voltages making the device suitable for use in applications having explosive atmospheres.
Another object is to provide a method for the non-destructive testing of the integrity and readiness of the actuator circuit for a separation device of the type described.
Another object is to provide a method of calculating the time of release in a separation device of the type described.